Chair base



FIG. 1 is a perspective view of a chair base showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a right side elevational view thereof, the left side being a mirror image; and,

FIG. 6 is a rear elevational view thereof.

The broken lines illustrate portions of the chair base that form no part of the claimed design. 

CLAIM The ornamental design for a chair base, as shown and described. 